Tunable diode laser absorption spectroscopy (TDLAS) is a well-proven technique for detecting and measuring the concentrations of various species in a gaseous mixture. TDLAS relies on the unique absorption spectrum of the species being targeted to measure an attenuation of a diode laser beam at a very specific wavelength, tuned over an absorption line of the species being measured, as it passes through a measurement region. At wavelengths even slightly different from these absorption lines, there is essentially no absorption.
Generally, in operation the wavelength of the diode laser beam is scanned over a small range that encompasses at least one absorption line of the species of interest, as well as a region in which there is no absorption. The light intensities of the light transmitted through the sample are measured by a photodetector. The photodetector signals are then analyzed to obtain an average concentration of the target species over the length of the beam path with knowledge of the temperature.
Because the molecules of each target species absorb light at a particular frequency, a different diode laser is generally required to measure different species. For certain applications, TDLAS systems need to use wavelengths encompassing a wide wavelength range from approximately 760 nm for oxygen (O2) detection to 2.33 microns for carbon monoxide (CO) detection. Wavelength-multiplexed TDLAS systems exist that use wavelengths from 760 nm to 1559 nm with the 1559 nm wavelength used for CO detection. However, some applications require a lower detection limit for CO than can be achieved at 1559 nm. For instance, in coal-fired boiler applications, the measurement path length can be over 10 meters and in the combustion zone the CO concentration can range upward from 5000 ppm. Under these conditions, CO detection using the second overtone band in the 1550 nm region works adequately. Because this wavelength region is widely used in telecommunications, single-mode optical fiber with high transmission is readily available, along with robust fiber-optic components such as switches, lasers and splitters.
However, certain applications require detection capability for CO at much lower concentrations and over a much shorter path. For example, detection of CO in the range of 100 ppm over a 1 meter path involves measuring roughly a 500 times smaller effect on the light intensity of the transmitted beam, making detection substantially more difficult than in the coal boiler application. This necessitates measuring CO on the first overtone at about 2.33 microns at which CO has a transition line strength that is approximately 500 times higher than that at 1559 nm.
Recently, TDLAS systems have been deployed that operate at wavelengths from 1350 nm to 2 microns. The extension to 2 microns allows sensitive detection of CO2 for carbon balance determination in steel applications. In many steel applications, O2 need not be measured so the 760 nm wavelength is not required. In spite of this wavelength range extension, the same single-mode fiber can be used to transmit, in single-mode fashion, light in this entire wavelength range (1350 nm-2000 nm). However, further extension to about 2.33 microns for sensitive CO detection, and a requirement to measure O2 at 760 nm necessitates a completely different architecture due to the inability of a single type of single-mode fiber to deliver wavelengths from about 760 nm to 2.33 microns single mode with high transmission and low bend loss.
Since light at wavelengths from about 760 nm-2.33 microns cannot co-propagate on the same single-mode fiber, a new wavelength multiplexing scheme must be devised for applications requiring both O2 detection and sensitive CO detection. One such application is glass furnace monitoring. Three wavelengths are required to measure O2, water (H2O), and CO for this application with CO detection required at the 100 ppm level or lower. Also required for this application is the ability to measure across approximately 10 paths simultaneously, or nearly simultaneously.